In many applications, it is often necessary to duplicate several times the same radio frequency (RF) circuit or functional block. Each duplicated circuit is designed with a different electrical characteristic to optimize performance in a limited range of radio signal frequencies. The circuit or functional block that is the most adequate to the current operating conditions is automatically selected by closing and opening switches and/or commanding path selectors that choose one path among several available paths.
A general application of such a switching scheme may be described by referring to FIG. 1. This figure shows N amplifiers A1-AN, and N groups of three switches SA1-SAN, SB1-SBN, SC1-SCN. Each group of switches is associated to a respective amplifier A1-AN. Each of the N amplifiers and the groups of switches can be connected to a capacitor C. The switches function as selectors for exclusively connecting one of the N amplifiers to the capacitor C.
For example, the amplifier A1 is selected through the SA1, SB1 switches and the capacitor C is coupled to the selected amplifier A1 through the SC1 switch. The scheme of FIG. 1 is applicable when the alternately operating N amplifiers require large valued capacitors for their correct functioning. Large valued capacitors may be needed to provide a high level of stability to the N feedback amplifiers, and to reduce the pass band for lowering the noise power. In these cases, the most immediate approach of using a capacitor for each amplifier would require a large silicon surface area for integrating all the capacitors. Alternatively, N external capacitors would be used if integrating the capacitors was too costly and/or not practical. This latter approach requires N dedicated pins.
The scheme of FIG. 1 represents the known approach of using a single capacitor that is selectively switched by closing one of the SC1-SCN switches to one of the amplifiers A1-AN, that is then selected through the respective SA1-SAN and SB1-SBN switches. The capacitor C may be integrated if convenient, or may be an external capacitor requiring only one dedicated pin. The use of external capacitors results in a cost reduction.
The scheme of FIG. 1 may be formed by using an alternative technique for selecting the amplifiers, instead of using the respective SA1-SAN and SB1-SBN switches. These techniques include leaving in an ON state the selected amplifier and switching OFF all the deselected amplifiers by controlling the bias current of the amplifiers. This approach is preferable under many conditions.
The circuit of FIG. 1 may be readily implemented in a CMOS technology due to the availability of analog switches that provide excellent performance with MOS transistors. In contrast, there may be several problems using bipolar technology. These problems are due to the use of analog switches formed with bipolar transistors operating between the cutoff and saturation regions.
A switch made with a bipolar junction transistor requires a relatively large base current to turn on the switch, and thus implies relatively long switching times. The main problem is due to the fact that a bipolar junction transistor may trigger parasitic transistors when dynamically switched between the cutoff and the saturation regions. This may negatively affect the functioning of the integrated circuit.